A progress in an electronic display as man machine interface has resulted in popularization of an interactive input system. Among others, an apparatus having a touch screen (a digitizer) and an associated display is widely used in various fields such as an ATM (automated teller machine), a merchandise management, an outworking (canvassing, selling), a guide sign, and an entertainment device. Use of the touch screen in combination with a lightweight and thin display (e.g., a liquid crystal display) dispenses with any keyboard and exhibits the features of the display, and thus such a touch screen display is increasingly used for mobile devices. The touch screen display is a device for inputting (or feeding) information or instructions to a computer by touching a prescribed area on a touch screen (or a touch sensitive panel) with an input means such as a finger or a pen (a digital pen or a stylus). According to the position detection method, the touch screen display can be classified into an optical system, an ultrasonic-wave system, a capacitive system, a resistive system, and other systems.
Among these systems, the capacitive system detects the location of the touch using a change in capacitance. In light of excellent functionality, a projected capacitive touch screen display of indium oxide-tin oxide-series compound oxide (ITO) grid system is now being used for mobile devices and is receiving a lot of attention. The mobile devices include a smartphone, a mobile phone, an electronic paper, a tablet personal computer (PC), a pen tablet (or a graphics tablet), and a game console. In particular, for a device such as the smartphone or the tablet PC, a high-definition display is beginning to be widely used. The display for such a device also requires optical characteristics such as high transparency or anti-glareness. Moreover, the following are also developed: a high-resolution (4K) touch screen television having a pixel dimension (or pixel number) four times as large as that of a full high-definition television, and a high-resolution pen-input device that is used in the architectural field or the medical field. A transparent material is used for a display surface (an upper transparent electrode) of a touch screen display of such a device. As the transparent material, a glass material is widely used in terms of excellent transparency or heat resistance. The glass material, which is brittle, easily breaks due to falling or other causes, scattering into fragments. It is thus necessary to take measures to prevent the scattering of fragments (or pieces) of the glass material if the glass material breaks. A known method for preventing the scattering of fragments produced (or generated) by breakage of the upper electrode (cover glass) composed of the glass material includes pasting a hardly broken plastic film (a film for preventing scattering of glass fragments) to the inside (backside or inner layer) of the cover glass.
An anti-shatter film is usually a laminate composed of, in sequence, a pressure sensitive adhesive layer [such as an optical clear adhesive (OCA) film] for attaching the anti-shatter film to a cover glass, a transparent resin layer such as a poly(ethylene terephthalate) (PET) film, and a clear hard coat (CHC) layer for protecting the transparent resin layer from scratches in the process of production or commodity circulation. For example, for the capacitive touch screen of ITO grid system, in which an ITO (indium oxide-tin oxide-series compound oxide) film is laminated on the inside (backside or inner layer) of a cover glass disposed on the outermost side of the touch screen, the pressure sensitive adhesive layer of the anti-shatter film is attached to the ITO film, and the CHC layer is disposed toward a display device (a display unit) such as a liquid crystal display (LCD) or an organic electroluminescence (EL) display (OLED). In some cases the CHC layer and the display device are bonded together through a transparent pressure sensitive adhesive layer or other means. In light of productivity or others, a popularized method includes interposing an adhesive layer (a spacer) between the edge (periphery or outer frame) of the CHC layer and that of the display device to bond the edge of the CHC layer and that of the display device with a space (a space layer) between the hard coat layer and a liquid crystal layer of the display device. Unfortunately, for the capacitive touch screen of ITO grid system, which has such a space layer (a space), touching the display surface with a finger or a pen may keep the CHC layer adhering to the surface of the display device (e.g., a polarization layer in an LCD) and show blacking phenomenon (black spot). This phenomenon is referred to as water marks (WM). In order to prevent or reduce this phenomenon (WM production), for a conventional touch screen, a particle having a size of the order of micron is mixed to a hard coat layer to form an anti-water-mark (AWM) layer having an uneven structure with recesses and/or protrusions of the order of micron, and adhesion of the AWM layer to the surface of the display device is prevented. Along with a recent high-definition touch screen display, however, an anti-shatter film having a particle-containing AWM layer causes sparkling which reduces the visibility. In particular, higher protrusions of the uneven structure on the one hand achieve more improved AWM characteristics and on the other hand, easily cause sparkling. There is a trade-off relationship between the AWM characteristics and the optical characteristics.
As a functional layer for being disposed in a display device such as an LCD, a functional layer having an uneven structure on a surface thereof has been reported. Japanese Patent No. 5170635 (JP-5170635B, Patent Document 1) discloses a process for producing an optical laminate comprising a substrate having light transmittance and an anti-glare layer on the substrate, the process comprising forming an uneven shape having a mean spacing of profile irregularities (or an average spacing of unevenness or an average spacing of concavo-convexes) Sm of 100 to 600 μm, an average slope angle θa of 0.1 to 1.2°, and a ten-point average roughness Rz of more than 0.2 μm and not more than 1 μm on the anti-glare layer. According to the document, in order to achieve both prevention of sparkling and improvement of contrast for a high-definition display such as a cathode ray tube display (CRT) or an LCD, the optical laminate is disposed so that the uneven shape is located at the outermost side.
The document is, however, silent on a touch screen and is also silent on scattering of fragments of a broken glass substrate. Moreover, if the optical laminate is used as an anti-shatter film, sparkling would appear on a high-definition display provided with the optical laminate.
WO2011-108394 (Patent Document 2) discloses an optical film comprising a transparent film and a hard coat layer disposed on the transparent film, wherein the hard coat layer comprises a cured product of a curable resin precursor, a thermoplastic resin, and a metal oxide fine particle having an average primary particle size of 1 to 60 nm. The document discloses that an uneven structure having a roughness average (or an arithmetic average roughness) Ra of 0.03 to 0.15 μm and a mean spacing of profile irregularities Sm of 50 to 300 μm is formed on the surface of the hard coat layer. The document describes use of the optical film for a resistive touch screen for the purpose of improving anti-Newton-ring characteristics. In Examples of the document, a hard coat layer having a roughness average Ra of less than 0.03 μm has no anti-Newton-ring characteristics and is stated as a comparative example.
The document is also, however, silent on any relationship between the hard coat layer and the breakage of a glass material. Further, if the optical film is used as an anti-shatter film, sparkling would appear on a high-definition display provided with the optical film.